Make separate debug symbols opt-in

This adds a separate_debug_symbols option to the x11, windows, and osx
targets. This will default to adding normal debugging symbols to the
artifacts and only splits them when separate_debug_symbols=yes on the
Scons command line.

platform/windows/detect.py

@@ -69,6 +69,7 @@ def get_opts():
         # Vista support dropped after EOL due to GH-10243
         ('target_win_version', 'Targeted Windows version, >= 0x0601 (Windows 7)', '0x0601'),
         EnumVariable('debug_symbols', 'Add debug symbols to release version', 'yes', ('yes', 'no', 'full')),
+        BoolVariable('separate_debug_symbols', 'Create a separate file with the debug symbols', False),
     ]